MACOMB COUNTY, MI – This suburban Detroit county, hard hit by auto industry troubles and the economic downturn throughout Michigan, may well chart the path to victory for either Mitt Romney or Rick Santorum in Tuesday’s primary.

Macomb County is the famed epicenter of “Reagan Democrats” in America -- the mostly white, blue-collar, Catholic voters known not only for their conservative sympathies, but dogged independence and a tendency of swinging from party to party.

The idea behind the label was always conceptual, but more and more, these voters find themselves more at home with the GOP. And thanks to Michigan's semi-open primary, which allows same-day party registration, their voices will be heard Tuesday.

Macomb is a county where voters are maybe the most attentive to the Republican candidates’ message on jobs right now.

“Our area, we’re a manufacturing county -- pretty much middle class. It’s a blue-collar county and it’s been about jobs,” said Republican state Rep. Ken Goike, who represents a portion of the county in the Michigan state legislature. “You can just feel the devastation.”

This county is known as a bellwether in national elections. Ronald Reagan won it twice by large margins, as did George H. W. Bush in 1988 and 1992. Bill Clinton carried the county in his re-election bid, and Al Gore narrowly bested George W. Bush here in 2000, though Bush eked out a victory over John Kerry in 2004. Barack Obama beat Arizona Sen. John McCain by 8 percentage points in the 2008 election.

Penny Phelan, of Harrison Township, was driving around with her husband, James, on Sunday afternoon to place yard signs in support of Rick Santorum at busy intersections. The couple lost their home and has been living in an apartment after Penny lost her job as a kitchen and bath designer. James, an ex-Marine, works as a machinist.

A former Democrat who switched parties in the early 1980s, Penny said she felt that Democrats “have moved away from core values that are very important in families.” They’re backing Santorum not primarily because of his stance on moral issues, but because of his manufacturing agenda, and a sense that he’s more in touch with the people of Macomb than Romney, the native son of Detroit who left to start Bain Capital, later becoming governor of Massachusetts.

“I feel like Santorum is for the basic guy. He has that connection, and understands the average worker in manufacturing and things like that have value,” she said of the former Pennsylvania senator, adding of Romney: “I just don’t think he’s lived it. And that makes a big difference, when you have people in your life who are suffering, and it’s close to you.”

But Romney has undeniable strength in the area. Many older voters can recall the days when his father, George, served as governor. And Romney has taken every stride possible to remind voters of his roots in the area, in stump speeches and frequent TV advertisements.

Jared Maynard, a former chairman of the Macomb County Republican Party who is backing Romney, said that the former governor’s jobs-oriented message should play well on Tuesday.

“The governor just needs to keep doing what he’s doing now, just talking to people about jobs,” Maynard said at a table at Crews Inn, overlooking the water in Anchor Bay. “And whether you’re the pauper on the street or the man in the mansion or you’re someone in between -- when you talk about jobs, you’re speaking everybody’s language.”

Both candidates have courted these Regan Democrats in their own ways. And they’ve both succeeded and failed, to some extent.

Santorum’s speeches are winding and sometimes disorganized, evinced by his Friday evening address in Lincoln Park. The speech was billed as a major address to outline a 10-point plan for his first 100 days as president, but these points became garbled, leaving some searching for crux of his message.

Rick Santorum and Mitt Romney are making the rounds with Michigan voters on Monday, trying to overcome self-inflicted slip-ups and win a state that could throw this Republican primary into a tailspin.

And Romney has reminded these middle class voters of his immense wealth by noting that his wife owns two Cadillacs during a Friday economic speech at Ford Field, the home of the Detroit Lions. And during a trip Sunday to the Daytona 500, meant to emphasize his love for cars, Romney said he has “great friends who are NASCAR team owners."

The county has changed in a number of ways since Regan was president. Its population has grown, but the average age of the population has also risen. It’s a much more diverse place, too, driven in part by Detroiters leaving the city.

But Macomb’s most defining characteristic remains its economy. The unemployment rate stood at 9.6 percent at the end of 2011, but that actually marked a decline from the recent height of a startling 17.3 percent jobless rate in June of 2009, at the height of the troubles in the auto industry. The struggles of automakers have contributed in part to a decline in household incomes over the last decade.

“When I did move over here, it was an interesting culture of people who were very attached to the auto industry -- a well-established middle class,” said Kathy Vosburg, the first Republican chairwoman of the Macomb County Board of Commissioners. “Many times they had different views and more independent thinking. And I think a lot of that has to do with Macomb County being a bellwether.”

John Makely / msnbc.com

Penny and James Phelan drive through Macomb county placing several Rick Santorum signs near busy intersections before the Michigan primary election on tuesday.

That was true in the Republican primary of 2008, when Romney won 45 percent of the vote in Macomb on the strength of a message focused on his roots in the area, and a plea for assistance from the federal government. McCain won 25 percent of the county, and former Arkansas Gov. Mike Huckabee took 13 percent of the primary vote.

No polling has been conducted specifically of Macomb leading into Tuesday’s primary, though an EPIC/MRA survey last week found that Romney led in the three combined counties of Macomb, Oakland (a more liberal suburb) and Wayne (which encompasses Detroit).

Given Romney’s built-in advantages, the former Massachusetts governor’s campaign should be alarmed if they start seeing signs of Santorum inroads during returns tomorrow night.

“If you see Santorum coming in Macomb or Oakland getting above 30 percent, that should be something to worry the Romney folks,” said Bernie Porn, the president of EPIC/MRA.

“What would surprise me is if anyone wins by more than 5 or 6 percent,” said state Rep. Pete Lund, Republicans’ majority whip in the state House who represents part of Macomb.

Presidential candidate Rick Santorum touts his "positive message of hope" on jobs and the economy in the upcoming Michigan primary.

Republican State Sen. Jack Brandenburg , who voted for Santorum but has not publicly endorsed in the race, said things look good for the former Pennsylvania senator.

“In the Republican primary, between Romney and Santorum, it’s going to go down to what faction gets their people out to vote. In Macomb County, I think the conservative element will come out strong -- and that bodes well for Rick Santorum,” he said, cautioning, “Romney has a lot of dough and a tremendous amount of resources.”

Both Santorum and Romney are stumping throughout the state of Michigan in the closing hours of the campaign to make their closing argument. Romney, speaking Monday morning in Rockford, challenged Santorum to speak more directly about the economy. That follows a weekend in which Santorum hit Romney for not focusing enough on industry in the state.

And come Tuesday’s GOP primary, whichever candidate can best speak to voters on that issue may end up carrying Macomb – and, with it, the rest of the state.